Ahead of the battle, the Iraqi army dropped tens of thousands of leaflets over Mosul urging residents to hide before the offensive begins.

The leaflets carried several messages to the citizens of the city in northern Iraq attempting to reassure them that the advancing army "would not target civilians" but warned them to avoid the known locations of Isis militants.

"Keep calm and tell your children that it is only a game or thunder before the rain," a leaflet said. "Women should not scream or shout, to preserve the children's spirit."

"If you see an army unit, stay at least 25 metres away and avoid any sudden movements," another said.

They have previously forced civilians to remain where they are rather than flee during previous battles to maintain territory.

The offensive to "liberate" Mosul from ISIS has begun. CNN’s Arwa Damon is near Mosul and describes what she's seeing pic.twitter.com/3Zskk5WzGI

It comes just hours after they lost control of the symbolically important town of Dabiq in Syria to a group of rebels after a short battle.

The opposition fighters were able to retake the town with "minimal" resistance despite Isis prophecies that Dabiq was to be the scene of an apocalyptic final battle between Muslims and Christians.

Isis has lost a third of its territory in Iraq and Syria in the past year as counter insurgent forces and air strikes push the group back.

Last week, the UN warned that it was bracing itself for the world's biggest and most complex humanitarian effort following the battle which it expects to displace up to one million people and see civilians used as human shields or even gassed.